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TheCarnian pluvial episode (CPE), often called theCarnian pluvial event, was a period of major change inglobal climate that coincided with significant changes in Earth'sbiota both in the sea and on land. It occurred during the latter part of theCarnian Stage, the first subdivision of theLate Triassic Epoch, and lasted for perhaps 1–2 million years (around 234–232 million years ago).^[6][7] Volcanic activity off the coast of North America led to global warming and increased rainfall on land, alongside a reduction ofcarbonate platforms in the oceans.Pluvial means "of or relating to rain; characterized by much rain, rainy."^[8]

The CPE corresponds to a significant episode in the evolution and diversification of manytaxa that are important today. The earliestdinosaurs (which include the ancestors of birds),lepidosaurs (the ancestors of modern-day lizards, snakes, and thetuatara) and potentiallymammaliaforms (ancestors of mammals) all diversified during the event. In the marine realm it saw the first appearance among the microplankton ofcoccoliths anddinoflagellates,^[9][7][10] with the latter linked to the rapid diversification ofscleractinian corals through the establishment of symbioticzooxanthellae within them. The CPE also saw the extinction of many aquaticinvertebrate species, especially amongammonoids,bryozoans, andcrinoids.^[6]

Evidence for the CPE is observed in Carnianstrata worldwide and in sediments of both terrestrial and marine environments. On land, the prevailing arid climate across much of thesupercontinentPangea shifted briefly to a hotter and more humid climate, with a significant increase in rainfall and runoff.^{[6][11][9][12][13]} In the oceans,carbonate minerals such aslimestone saw reduced deposition, leaving mud-rich layers as prominent geological markers. Carbonate disruption may reflect the extinction of manycarbonate-forming organisms, but may also be due to a rise in thecarbonate compensation depth, below which most carbonate shells dissolve and leave few carbonate particles on the ocean floor to form sediments.^{[14][15][16][17]}

Climate change during the Carnian pluvial event is reflected in chemical changes in Carnian strata across the CPE. Major disruption to thecarbon cycle and other natural systems show thatglobal warming was prevalent at the time. This climate change was most likely linked to the eruption of extensiveflood basalts and volcanicCO2 offgassing as theWrangellia Terrane wasaccreted onto the northwestern end of theNorth American Plate.^[11]

Environmental disturbance and high extinction rates were observed for sediments of the Carnian stage long before a global climate perturbation was proposed. Schlager & Schöllnberger (1974) drew attention to a darksiliciclastic layer which abruptly interrupted a long period ofcarbonate deposition in theNorthern Limestone Alps.^[18] They termed this stratigraphic "wende" (turning point) the Reingrabener Wende, and it has also been called theReingraben event orRaibl event.^[15][19] Several Carnian terrestrial formations (namely theSchilfsandstein ofGermany and various members of theUnited Kingdom'sMercia Mudstone Group) are intervals of river sediments enriched withkaoliniticclay and plant debris, despite having been deposited between more arid strata. Humidity-adapted palynomorphs inNew Brunswick,karst topography in the U.K., and acarbon isotope excursion inIsrael were all reported for the middle of the Carnian prior to 1989. The Julian-Tuvalian boundary experienced high extinction rates among many marine invertebrates, while an extinction among land vertebrates was suggested to occur in the late Carnian.^[6]

In 1989,Michael J. Simms andAlastair H. Ruffell combined these disparate observations into a new hypothesis, pointing to an episode of increased rainfall synchronous with significant ecological turnover in the mid-Carnian.^[6] The paper was inspired by a conversation between Simms and Ruffell, on 10 November 1987 at Birmingham University, that connected Ruffell's research onlithological changes in the Mercia Mudstone Group to Simms's research oncrinoid extinction.^[20] A key aspect of their hypothesis was that the evidence used to demonstrate the climate change was entirely independent of the evidence for biotic change; fossils were not used in any way to infer climate change. Their hypothesized climatic disturbance, which they named theCarnian pluvial episode, was tentatively considered to be a result of oceanic and/or volcanic instability related to the early rifting of Pangea, but at that time direct evidence of this possibility was lacking.^[6]

Simms and Ruffell published several more papers in subsequent years,^[21][22] but their hypothesis was not widely accepted.^[20] A strong critique by Visscheret al. (1994) argued that aridity-adapted pollen stayed abundant through the entire Carnian of Germany, suggesting that the Schilfsandstein was simply indicative of an invading river system rather than widespread climate change.^[23] Their critique also coined the term "Carnian pluvial event", which would eventually become among the most widespread names for the climatic disturbance.^[17][24]

The obscurity of Simms and Ruffell's hypothesis began to dissipate in the late 2000s, as further support accumulated from studies on Carnian sites inItaly.^[17][25][20] Interest in the hypothesis was greatly enhanced by a 2008 meeting and workshop on Triassic climate at theMuseum of Nature South Tyrol inBolzano, Italy.^[24][20] However, even as the global nature of the CPE became increasingly accepted, its ultimate cause was still hotly debated going into the 2010s. Even its nomenclature was not agreed upon, with various authors applying names such as themiddle Carnian wet intermezzo,^[26][27]Carnian humid episode,^[21][28][29]Carnian pluvial phase,^[30][31] andCarnian crisis.^[32] Carbon andosmium isotope records published over the coming years supported a strong link between the Carnian climate disturbances and the Wrangellia large igneous province, but many questions remain unanswered.^[33][11] A geological workshop focusing on the CPE met in 2018 at theHanse-Wissenschaftskolleg (HWK) Institute for Advanced Study inDelmenhorst, Germany. The workshop was intended to spur further research on the mechanisms, impact and stratigraphy of the CPE, as well as its relevance for understanding modern climate change. It also attempted to standardize the nomenclature of the CPE; rejecting descriptors such as "event" (typically applied to geological processes under a million years in duration) or "middle Carnian" (a nebulous term with no equivalent geological substage).^[34]

The Carnian pluvial episode introduced markedly more humid conditions across the globe, interrupting the otherwise arid climate of the Late Triassic period. This humidity was related to increasedrainfall during the CPE, evidenced by:

• Siliciclastic (highsilica-content) sediment insedimentary basins, reflecting a high level of continentalweathering andrunoff;
• Significantkarst conduits (caves) in Palaeozoic limestone inliers beneath the Late Triassic terrestrial unconformity. The topographic context of these caves is consistent with a Carnian age,^[35][36] although some research groups instead support a Rhaetian age based on localised occurrence of microfossils.^[37]
• The development ofhistic andspodicpalaeosols, fossil soils which are typical of atropical humid climate with more water entering throughprecipitation than leaving throughevapotranspiration;
• Hygrophyticpalynological (fossil pollen) assemblages that reflect vegetation more adapted to a humid climate;
• The widespread presence ofamber.^[38]

This usually wet climate of the CPE was periodically interrupted by drier climates typical of the rest of the Late Triassic period.^[30] One climate simulation argues that the interior of Pangaea actually became drier during the CPE, even as its eastern margin and high-latitude regions became rainier. The onset of the episode may have been very rapid (~15,800 years), amplified by carbon cycle feedback effects.^[39]

Global warming was also prevalent during the Carnian pluvial event. This is evidenced by oxygenisotope analyses performed onconodontapatite from the CPE, which show an approximately 1.5‰ negative shift in the stable isotopeδ¹⁸O. This result suggests global warming of at least 3–4 °C during the CPE and/or a change inseawatersalinity.^[32][40] Wider sampling supports warming on the order of 4–8°C.^[29][9][41] This warming was almost certainly related to extensive volcanic activity at the time, evidenced by carbon isotope trends across the CPE.^[11] This volcanic activity was in turn probably related to the formation of theWrangelliaLarge igneous province around the same time, which created vast quantities ofigneous (volcanic) rocks that wereaccreted onto the northwest end of theNorth American Plate (now theWrangell Mountains,Alaska, andan estimated 6km thick layer underlying most of Vancouver Island).^[11]

Chemical weathering intensified during the CPE, according toLithium isotopes in volcanic lake ash in North China. Prior to the event, lithium is found in low concentrations, mostly sourced from airborne ash in a cool and dry climate. During the CPE, river input becomes a greater influence on the system, with the hot, wet climate increasing the amount of lithium weathering out of ash beds on land.^[42] Weathering was also seen in prehistoric coastal Europe, enhancing runoff of terrestrial sediments responsible for local mud-rich marine layers. High rates of weathering continued even after the earth returned to a drier climate, suggesting that the CPE eroded enough sediment in the region to exposebasement rock.^[43]

The onset of the CPE marks a sharp change in the shape and composition ofcarbonate platforms across the entireTethys Ocean, which extended from Central Europe to East Asia.^{[15][44][45][29]} The early Carnian was characterized by high-relief, mainly isolated, small carbonate platforms surrounded by steep slopes. During the CPE, these were replaced by low-relief carbonate platforms with low-angle slopes (i.e., ramps). This turnover is related to a major change in the biological community responsible for calcium carbonate precipitation (in other words, the "carbonate factory"). High-relief platforms relied on a highly-productive biological community of carbonate-secreting microbes. When microbial carbonate declined during the CPE, a less productive animal-dominated carbonate community was left to pick of the slack, leaving skeletal grains (shell fragments) andooids as the primary component of limestone.^[46][47]

In many regions, rain and river activity on land led to an increased flow of sand, silt, mud, and clay into the ocean. These terrestrialsiliciclastic sediments form distinct, easily-eroded layers in areas otherwise dominated by carbonate.^[17][48] Carbonate deposition is eliminated altogether in rare deep marine sediments found in Southern Italy. Thecarbonate compensation depth (the minimum depth where calcium carbonate dissolves completely) was probably shallower during the CPE than before or after.^[17] In Turkey, which was near the equator during the Carnian, the demise of carbonate platforms was delayed compared to more northerly seas.^[44]

In the SouthChina block, the demise of carbonate platforms is coupled with the deposition of sediments typical ofanoxic environments (blackshales). Thanks to low deep-water oxygen levels, animal remains were often well-preserved in sedimentary deposits calledlagerstätten. These lagerstätten are rich in crinoids and reptiles such asichthyosaurs.^[49] Some of the fossil-rich layers overlap with the Carnian Pluvial Episode, such as the transition from theZhuganpo toXiaowa formations.^[45][29] Nevertheless, black shales are not as abundant in the CPE compared to other extinctions (which typically coincide withoceanic anoxic events).

There is some evidence for seabedeuxinia (no oxygen and high toxic sulfide concentrations) during the CPE. Limestones are enriched inmanganese ions near the top of the Zhuganpo Formation. Manganese ions are concentrated and soluble in deep euxinic waters, butprecipitate in carbonates at the base of the oxygenated zone. Increasing manganese concentrations indicate a narrowing of the oxygenated zone and a corresponding expansion of euxinic water.^[29]

• 
  Marine extinctions^[9]
• 
  Some of the major biological changes^[9]
• 
  Tetrapod footprints mapped against Alpine stratigraphy, biozones, and climate in the Triassic^[50]

Conodonts,ammonoids,crinoids,bryozoa andgreen algae experienced high extinction rates during the CPE. Other organisms radiated and diversified during the interval, such asdinosaurs,calcareous nannofossils,corals andconifers.^{[6][21][22][9]}

Some studies interpret the CPE as a key geobiological event allowing dinosaurs to diversify.^[50][51][52] The oldest well-constrained geological units with dinosaur fossils are theSanta Maria Formation ofBrazil and theIschigualasto Formation ofArgentina. The latter's earliest dinosaur-bearing layers are radiometrically dated back to 230.3 to 231.4 million years ago. This is similar to early minimum age estimates for the CPE (≈230.9 million years ago). More recent studies place the CPE a few million years earlier, near the start of the underlyingLos Rastros Formation.^[51]

Comparisons of tetrapodichnofossils (footprints) from before, during, and after the CPE suggest an explosive increase in dinosaur abundance due to the Carnian humid phase.^[50] However, whileavemetatarsalian diversity, diversification rate, and size disparity does increase through the Carnian, it increases faster in the Ladinian and Norian, suggesting that the CPE was not a major influence on the rise of dinosaurs.^[53] Precipitation has no apparent correlation with dinosaur diversity across the Late Triassic, with latitude as a better proxy instead.^[54]

The CPE had a profound effect on the diversity andmorphologicaldisparity (shape variety) of herbivorous tetrapods.^[55] This is exemplified inrhynchosaurs, a group of reptiles with strong shearing and grinding jaws. Rhynchosaur lineages which were common in the Middle Triassic went extinct, leaving only the specializedhyperodapedontines as representatives of the group. Immediately after the CPE, hyperodapedontines were widespread and abundant in the late Carnian world, suggesting that they benefited from the climate fluctuations or floral turnover.^[56] Hyperodapedontine abundance was not sustained for long, and they too would die out in the early Norian. By cutting rhynchosaurs off from more generalized niches, the CPE would have reduced their versatility and increased their long-term vulnerability to extinction. Similar trends are observed indicynodonts, though they would survive until much later in the Triassic. Conversely, more versatile and generalist herbivores such asaetosaurs andsauropodomorph dinosaurs would diversify after the CPE.^[55]

Some studies argued that mammals originated during the CPE.^[9] More precisely, "mammal" refers tomammaliaforms (extremely mammal-likecynodonts, appearing prior to the common ancestor of modern mammals). The oldest claimed mammaliaforms areTikitherium (from India) andAdelobasileus (from Texas). However, both are likely younger than the Carnian:Tikitherium appears to be a misidentifiedCenozoic shrew,^[57] whileAdelobasileus is "no older than 225 Ma".^[58] Mammaliaforms and their closest relatives, the buck-toothedtritylodonts, together make up the groupMammaliamorpha. Mammaliamorphs were the first fullyendothermic cynodonts, and their ancestry can be traced back to the CPE.^[59] In the subsequentNorian stage, unambiguous mammaliaforms appeared on the scene, withmorganucodonts,haramiyids, and other forms throughout Europe and Greenland.^[9]

Rhynchocephalians (relatives of the moderntuatara) achieved a worldwide distribution by the end of the Carnian.Crocodylomorphs,phytosaurs, andturtles also began to diversify after the CPE.^[9] There is some ambiguity regarding a cause-and-effect relationship between the CPE and terrestrial diversification events, many of which were prolonged processes through the Middle and Late Triassic.^[60]

Conifers,ferns, and the now-extinctbennettitaleans all diversified greatly during and after the CPE, establishing themselves as mainstays of Mesozoic flora. Most regions show a higher proportion of hygrophytic (moisture-loving) plants during the episode compared to earlier parts of the Triassic.Spores of ferns and freshwater algae are frequently abundant inpalynological samples. The Carnian saw the reestablishment of large inland lakes andpeat swamps, ending the Early-Middle Triassic "coal gap" caused by the Permian-Triassic mass extinction. Increased plant growth and coal burial probably helped to draw down CO₂, returning the atmosphere to a more normal state after the CPE.^[61]

Though tinyamber traces can be found in rocks as old as theCarboniferous, the earliest widespread amber deposits date back to the CPE.^[38] Carnian amber droplets from Italianpaleosols are the oldest amber deposits known to preservearthropods andmicroorganisms.^[62] Amber would not reappear in the fossil record until theLate Jurassic, though it would take until theEarly Cretaceous for amber to occur in concentrations equivalent to or exceeding Carnian amber.^[63][38]

Radiolarians increased in their diversity, likely as a result of increased continental weathering amidst the warmth and humidity of the CPE.^[64] The firstplanktonic calcifiers occurred just after the CPE and might have been calcareous dinocysts, i.e., calcareouscysts ofdinoflagellates.^[9] Foraminifera saw no global extinction across the CPE, apart from a localized decline in thePaleotethys.^[65]

Coastalostracod communities in Hungary experienced major changes across the CPE. Through Julian 2, land-based sediments isolated and filled in marine basins, replacing carbonate-specialists such asbairdiids andhealdiids withBektasia, aplatycopid tolerant of shallow siliciclastic seas. Further shallowing across the Julian-Tuvalian boundary left only a few aberrantlimnocytherids (Renngartenella,Simeonella) andcytherurids (Kerocythere) which could manage severe salinity fluctuations in the restricted coastal basins. Bairdiids returned in force at the end of the crisis when the basins deepened, reacquiring carbonate and better ventilation.^[66]

The consensus cause of the Carnian Pluvial Episode is theWrangelliaLarge Igneous Province (LIP), a large patch of volcanic activity in thePanthalassan Ocean. In the present day, these volcanic rocks have beenaccreted onto Alaska and British Columbia. The recent discovery of a prominentδ¹³C negative shift in higher plants' n-alkanes suggests a massiveCO₂ injection in theatmosphere-ocean system at the base of the CPE. The minimum radiometric age of the CPE (≈230.9 Ma) is similar in age to thebasalts of the Wrangellia LIP. In the geological record, LIPvolcanism is often correlated to episodes of major climate changes and extinctions, which may be caused by pollution ofecosystems with massive release of volcanic gases such as CO₂ andSO₂. Conodont biostratigraphy and magnetostratigraphy clarifies the timing even further to a start around 234.5 Ma.^[67]

Large release of CO₂ in the atmosphere-ocean system by Wrangellia can explain the increased supply of siliciclastic material into basins, as observed during the CPE. The increase of CO₂ in the atmosphere could have resulted inglobal warming and consequent acceleration of the hydrological cycle, thus strongly enhancing the continentalweathering. Moreover, if rapid enough, a sudden rise of pCO₂ levels could have resulted inacidification of seawater with the consequent rise of thecarbonate compensation depth (CCD) and a crisis of carbonate precipitation (e.g. demise of carbonate platforms in the westernTethys). On top of all that, the global warming brought on by the flood basalt event was likely exacerbated by the release ofmethane clathrates.^[68]

The CPE is marked by disruptions togeochemical cycles, most notably thecarbon cycle. Sediments corresponding to the base of the episode show a prominent –2 to –4‰δ¹³C excursion, indicating the release of a lightweight carbon isotope,carbon-12, into the atmosphere.^[47] This excursion was first mentioned for carbonates in Israel,^[6] and was later reported in more detail from fragments of carbonized wood in the Dolomites.^[11] It has been confirmed in various carbon-based sediments throughout Europe and Asia.^{[47][29][69][70][67]}

More precise stratigraphic evaluation of European outcrops has resolved this excursion into three or possibly four major pulses, spanning the late Julian and early Tuvalian. Each pulse can be equated with an interval of abnormal sedimentation on land and sea. The third excursion, at the Julian-Tuvalian boundary, is correlated with major ammonoid and conodont extinctions.^[48] A four-pulse episode is also strongly supported in terrestrial systems, particularly lake and river sediments in North China.^{[41][71][72][73]}

Norwegian shale andJapanesechert from the Ladinian-Carnian boundary show a marked change in the ratio of seawaterosmium isotopes. The relative abundance of osmium-187 over osmium-188 declines strongly through most of the Julian before rebounding and stabilizing in the Tuvalian. The decline is attributed to early phases of the Wrangellialarge igneous province enriching the ocean with osmium-188. Osmium-188 is preferentially sourced directly from the mantle, while osmium-187 is aradiogenic isotope supplied from eroded land.^[33][74][75]

In theAlps, moderate to high concentrations ofmercury occur alongside carbon cycle disruptions, just prior to the sediment disruption which marks the CPE. These mercury spikes occur in well-oxygenated mudstones, meaning that they are not a consequence ofredox fluctuations. The ratio of mercury to organic carbon is stronger and occurs earlier in areas corresponding to open marine environments. Although the mercury spikes do not correlate with any indicators of terrestrial runoff, runoff could help maintain high mercury concentrations in the ocean through the CPE. The most parsimonious explanation is that the mercury was initially derived from a pulse of volcanic activity, particularly the Wrangellia LIP. This further supports a volcanic cause of the Carnian pluvial episode.^[76] Mercury spikes are also found simultaneous to carbon cycle disruptions in both marine^[77] and lake^[41][73] sediments in China, and marine strata in Japan.^[78] These mercury spikes have no trace ofmass-independent fractionation, meaning that their isotope distribution is most consistent with fallout from volcanic eruptions.^[77]

According to an alternative hypothesis, the Carnian pluvial episode was a regional climatic perturbation mostly visible in the western Tethys and related to theuplift of a newmountain range, theCimmerian orogen, which resulted from the closing of a Tethyan northern branch, east of the present European continent.

The new mountain range was forming on the southern side ofLaurasia, and acted then as theHimalayas andAsia do today for theIndian Ocean, maintaining a strongpressure gradient between the ocean and continent, and thus generating amonsoon. Summer monsoonal winds were thus intercepted by the Cimmerian mountain range and generated strong rain, thus explaining the switch to humid climate recognized in western Tethys sediments.^[32][15]

Conversely, erosion of mountain ranges in central Pangaea may have allowed more moisture to reach the interior of the continent.^[79]

In marine sediments ofTibet, a periodic pattern emerges from carbon isotope fluctuations, sea level highstand layers, and marine crises during the CPE.Astrochronology finds a strong link to a 1.2-million yearobliquity modulation cycle (the wobble in the tilt of Earth's axis). ThisMilankovitch cycle may have enhanced biotic turnovers in the marine realm.^[80] In theJunggar Basin, 405-kyreccentricity cycles (Earth's distance from the Sun) mark environmental fluctuations on land, similar to warm-cold climate cycles during theOligocene andMiocene.^[39] Eccentricity cycles are also apparent in the Jiyuan Basin of North China, with four evenly-spaced spikes of carbon and mercury isotope disruption over the course of 1.65 million years. The primary driver of the CPE would have been four pulses of volcanic activity in the Wrangellia LIP, withorbital forcing helping to amplify each pulse.^[73]

Coal swamps recovered in the early Carnian, about 15 million years after their demise in the Permian-Triassic mass extinction. Prior to the recovery, the Early and Middle Triassic was a time of low biological productivity. Global temperature, atmospheric CO2 levels, erosion, and soil oxidation were all very high. The return of the forests reversed the paradigm, establishing a productive pathway for sequestering CO2 into stable soils. According to one model, an early Carnian drop in global CO2 would increase the relative severity of the CPE. This is because CO2 has alogarithmic influence on earth's climate, so any new spike in CO2 levels (such as during the Wrangellia eruptions) would have a greater effect if it is preceded by lower background levels in the atmosphere.^[79]

• Carnian Stage
• Triassic extinctions
• Events that forced the climate
• Extinction events

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